Apparatus for controlling the temperature of fluid baths



July 22, 1941. R. c. UPTON 2,249,993

APPARATUS FOR CONTROLLING THE TEMPERATURE OF FLUID BATHS Filed May 1, 1939 INVENTOR.

RIC HARD C. UPTON ATTORNEYS Patented July 22, 1941 UNETED STATES FFECE APPARATUS FOR CONTROLLING THE TEM- PERATURE 0F FLUID BATES poration of Michigan Application May 1, 1939, Serial No. 271,129

2 Claims.

The invention relates to improvements in the control of the temperature of internally electrically heated fluid baths and more particularly molten salt baths used in metallurgical processes, such as the heat treating of steel and other metals.

It is one of the objects of the invention to minimize variations in temperature of the bath due either to variations in potential of the electrical energy supplying means, or to variations in electrical conductivity of the bath, or to variations in heat losses.

It is a further object to provide means for anticipating changes in electrical input required to compensate for the insertion of work into the bath, or its removal therefrom, so as to minimize temperature fluctuations.

With these and other objects in view, th invention consists in the method and construction as hereinafter set forth.

In the drawing:

Figure 1 is a diagrammatic view illustrating an electrically heated bath and my improved means for controlling the temperature thereof;

Figure 2 is a diagrammatic view showing a modification of the controlling means.

The invention makes use of the variations in the electrical conductivity of the liquid due to variations in its temperature. Since the electrical conductivity of the liquid is a simultaneous function of its temperature any device which is able to note variations in the electrical conductivity of the liquid directly notes the temperature changes therein, provided no other physical characteristics affecting its conductivity, except its temperature, are changing. Again, since the change in temperature of the liquid is the result of differences between the energy put into it by the heating agency and the heat lost by conduction and radiation of heat or supplied by the bath to colder materials added to it, if the device, in noting temperature changes in the manner noted above, is adapted to selectively controlling the energy input in accordance with the proper interpretation of such indicated changes, it is also adapted to controlling the temperature of the liquid or bath. If, furthermore, the device is adapted for effecting the addition of an amount of energy to the liquid correctly proportional to the addition of a predetermined amount of cold material to the same, it will be able to anticipate and prevent or minimize any change in temperature in the bath which would occur were the addition of the cold material made without simultaneously adding this proper amount of ener y.

With these considerations in mind, I have devised an apparatus constructed and organized as follows.

As shown in Figure l, l is the liquid bath heated by electric current passing through it between the electrodes 2 and 3, which are connected in series with the secondary winding of he transformer 4 supplied with energy from power lines 5 and 6 connected to its primary winding through a voltage regulator l operated by the reversible motor 3. This motor driven regulator l controls the energy input into bath 1 by controlling the primary voltage of transformer t. The secondary current in the bus bar which is the current passing through the bath, is linked with a magnetic circuit of a series-transformer (or current-transformer as it is usually spoken of) [Q by having the bus bar 3 pass through the window or opening in the iron circuit of the transformer. The secondary winding I do of the current transformer It has a series of taps l iii), Hie, etc, at diiierent numbered turns, and is connected in series with the coil of ammeter I l (which has a spring tension adjustment lever 38) through a tap changing switch I2. Thus, the current through bath I is proportional to the cur out through ammeter l l in a proper tionality determined by the setting of the tap changing switch l2. Resistance 13 prevents open circuiting of the secondary winding of'the current-transformer it, during the operation of changing the setting of tap changing switch l2.

shield 84 of the movable needle of the meter ii is limited in its swing by the stops l5 and it, so that its swing to the right for lower values of current limited by stop Hi, can be no greater than just enough to allow light from a light source ii to impinge on a photo-electric cell (8 through the full width of a slot [8 in a screen 2i This limit is necessary to prevent momentum being given to the needle by useless wide swings to the right during tap changing operations, since the maximum useful effect of any swing to the right, is obtained when the whole width of the slot ill is uncovered to the photo-electric cell it by the shield I l. The swing of the needle to the left is limited by stop i5, so that for higher values of current the slot I9 is completely covered by the shield I 4. This limit is necessary because wider swings than this, would again uncover the slot and expose to the photo-electric cell the same light conditions as obtained by too low current values. For varying values of current various amounts of light from light source ll will impinge on photo-electric cell 18 as the shield uncovers to it, various widths of light slot [9 in accordance with the movements of the ammeter needle corresponding to the various current values.

The electric output from the photo-electric cell [3 flows through the sensitive relay 2|. Low values of light on cell l8 result in low electric outputs from it, and when this output is low enough the contacts 24 and 23 of the sensitive relay 2| are closed, while high enough values of light on the cell will result in the closing of con tacts 23 and 22 of the sensitive relay, because the output of the cell [8 for these higher light values, will move contact 23 away from 24 and toward 22.

If the current through bath 1 be high enough so that with a given setting of the tap changing switch 12 and of meter spring adjustment lever 38 the shield 14 covers enough of the slot Hi to cause contacts 23 and 24 of sensitive relay 2! to close, the closing of these contacts will operate the relay 25, which will close a switch 26 causing the motor 8 to move regulator I, so as to lower the primary voltage of transformer i,

and thus lower the value of the current in bath I. This lowering will persist till the value of the current in bath l falls to such a point as to move shield [4 to the right enough to clear enough of slot [9 and to allow enough light from source I! to impinge on photo-electric cell l8 to suffir:

ciently increase its output so as to move contact 23 away from contact 24 and thereby stop the motor 8. Similarly a sufiiciently low value of current in bath I with the same (or other) setting of tap switch l2 and spring adjustment 3!), f

would result in the closing of contacts 22 and 23; the operation of a relay 21 and switch 28, thereby causing the running of the motor 8 in reverse direction. This will move regulator 1 so as to raise the primary voltage of transformer 4 till the value of current through the bath 1 becomes sufficient to cause, through the means above described, the stopping of motor 8.

Thus, the apparatus is seen to hold, for any given setting of tap switch [2 and spring tension lever 32, the value of current in bath l within upper and lower limiting values determined by the responsiveness of meter H, the sensitivity of relay 2! with respect to the output of cell [8,

and the intensity of light source IT, as well as its distance from light cell I8. Since these factors are adjustable and not subject to change due to natural causes, the combination may be said to be an accurate device for regulating a constant current through bath I.

The electrical conductivity of the bath depends on its temperature, but whatever this is, in accordance with the operation of this control as just described, the regulator I will be so adjusted automatically as to make the transformer 4 supply sufficient voltage to pass through the bath just that value of current required in accordance with the setting of tap switch l2 and lever 39. This value of curren multiplied by the value of voltage supplied to produce it, will be the amount of energy supplied to the bath, If this amount of energy is just equal to the heat losses from the bath, then the temperature of the bath will not change. If, however, this input be greater than the losses, the temperature of the bath will 1' rise; its conductivity increase; and the control apparatus will automatically reduce the voltage to keep the current value substantially constant. Thus, the input of energy will be lessened, but at the same time, due to the increased temperature the heat losses are greater. As the temperature increases adjustments of voltage will be automatically made lower until a condition of equilibrium is soon reached, where the lessened input equals the increased loss and a stable temperature is attained. In a like manner, if the input at first were less than the losses, the temperature would fall; the conductivity would be less; the voltage would be raised to maintain the current value; the input thereby increased, and the process would continue till the increased input equaled the lessened losses and equilibrium of temperature attained.

Thus, it is seen that this apparatus by reason of its being a constant current regulator, is a temperature regulator for the bath. For every setting of the tap changing switch l2 and the spring tension lever 39, there is a corresponding temperature to which the bath will be automatically brought. This depends on there being no physical changes in the bath nor in its relation to losses by conduction or radiation, factors which have to do only with the construction and maintenance of the furnace and nothing to do with the design of the control as such.

The addition of cold material to the bath will lower its temperature, resulting in the automatic control functioning as explained to bring the temperature back to the value called for by the setting of the switch [2. The voltage would be raised due to the lowered bath temperature and conductivity, in the proper amount, and for a long enough time to supply just that amount of heat taken from the bath by the cold added material. However, such a cycle of operations would take time in making the complete adjustment, but if at the time the material were added, the setting of tap switch I2 were changed so as to increase the value of the bath current and this changed setting were maintained for the proper length of time, the correct amount of energy would be added to the bath to supply the heat required by the added material to bring it to the bath temperature, and this without any delay due to waiting for the automatic temperature adjustment to be made by the control. In the salt bath method of heating material the flow of heat into the added material from the molten salt is very rapid and a piece of metal would arrive at the same temperature as the bath in which it is immersed sooner than this temperature could be automatically adjusted to its proper value after the lowering of the same by the addition of the cold material. And since the amount of the added energy in this operation is equal to the added rate of energy flow as represented by the increase in the setting of tap switch I2 multiplied by the length of time such increase in setting is maintained. it will be seen that if the time interval be fixed, then the added setting on switch !2 should be so much per pound of material added (dependent on the specific heat of the material and the increase in temperature given to it) while if the added setting on switch 12 be fixed, then the time interval of the duration of this setting will be so many seconds per pound of material, or a combination of these two methods may be used.

If material be added to the bath and then, when up to bath temperature or any approximation thereto, as in heat treating metals, it is removed; and if during this time the added enery were put into the bath in accordance with the method just described, then when the setting of switch 12 has been returned to the original temprature-holding setting end turns into circuit. the se ected time interval returned to contact with change i 5 setting on tap Was, ctive. This arholdi during heat "a ator that his proper amount. the rc-estao s it.

treating operations by keeping the two selectet settings separate so far as con- 'l a, so as a 1. m t. I! 4- t tlansicnnor t than ntroduced into the the 1 rnovcs r lee y vcltag it did before hath, such the th ttcc i contrc a suitable temperature ins thermal-couple within nersed in the bath and contasly calibrated miilivoltnieter, or the: suitable means. It is to be used Wis,

l mil-Z y and not as a controlling pilot, direction s on is not sensitive enough nor rapid ,1 too great when compared to the method herein mine-i. by e claim as invention is:

c rnoination with a fluid bath of the which electrical conductivity increases "einperature, and m ans for supplying elecu ergy thereto to internally heat the same, of in' ans responsive to variations in current, into changes in the bath from a predeternorrnal temperature and consequent electrical conductivity for varying the lied prac tiveiy con 5 i. The other t inal of "ed 30 pressed upon the electrical energy supcoinmon contact i the time switch 33 which as to vary the current so as to subswitch controls the time which .ntialiy 1r the normal temperature, incontact is t d. t st contact at the exd r adjustment means for selectively pirati cted interval contact 35 is ing the current value at which the aforerncved *nechamsm to make 95 eans is non-responsive, and automatic timcans operable on the introduction of work bath for first effecting said change in d subsequently restoring the normal val the total increase of heat generating contact a; u cc J'ie contact con nected pivot contact of rotary ct points are con- .d of the current ssncwn. P.,e

o to the t o" r i t r body absorbed by the Work in raising the temof the ec'ndary winding to re thereof to said predetermined normal preve "ature.

7 r The combination with an electrical furnace ap switch since 4%) or the fluid hath ty e and means for controlling eld together the the average rate of energy input to substantially h 533 are rot eithe temperature of the bath at a pret "he addition I eterrnincd point, of means including a timing ateriai added to mechanism operable upon the introduction of l. n tap switch 371. Work into the bath and independent of any tem- The operator then time switch 33 to eegin pereture changes therein for increasing the enerits cycle in accordance with a time decided upon gy input rate to a degree and for a time so that as correct. This changes contact rroin closure the total increase amount of energy substan- Wrth to closure with and brings the setially equals that absorbed by the work in raislected end turns connected to tap switch 31' into 55 ing the temperature thereof to said predetermined circuit causing the automatic control to raise point. the energy input by the predetermined amount RICHARD C. UPTON.

as required by the change in the number of ef- 

